output/PipeWire: add nullptr check, fixing crash with PipeWire 0.3.53

Since PipeWire 0.3.53, control names can apparently be nulled, leading
to crashes in applications assertion that the string cannot be
nullptr.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1558

NEWS

@@ -1,6 +1,8 @@
 ver 0.23.8 (not yet released)
 * storage
   - curl: fix crash if web server does not understand WebDAV
+* output
+  - pipewire: fix crash with PipeWire 0.3.53
 
 ver 0.23.7 (2022/05/09)
 * database

src/output/plugins/PipeWireOutputPlugin.cxx

@@ -235,7 +235,8 @@ private:
 				[[maybe_unused]] uint32_t id,
 				const struct pw_stream_control *control) noexcept {
 		auto &o = *(PipeWireOutput *)data;
-		if (StringIsEqual(control->name, "Channel Volumes"))
+		if (control->name != nullptr &&
+		    StringIsEqual(control->name, "Channel Volumes"))
 			o.ControlInfo(control);
 	}